This invention relates to cable systems that utilize amplifiers and other equipment at a limited number of distribution sites for providing cable signals to receivers at a plurality of subscriber stations. In particular, it relates to a cable system that has a built-in or self maintenance capability by virtue of the type of signals used.
All cable systems require periodic maintenance to insure delivery of proper signals over the full frequency range of the cable system frequency spectrum. All of the various receiver-amplifiers in the cable system should be periodically checked for distortion levels and frequency response. At present this is often accomplished by injecting a high (or low) level sweep signal into the channels of the cable band. A portable receiver of some type displays the sweep signal response at any receiver and appropriate adjustments in the signal from the distribution site or the cable head end are made as required. While the injected sweep signal interferes with the desired television signal, its effect is minimal and not objectionable for analog signals such as NTSC signals.
Many installed cable plants have a two-way communication capability, i.e. they have the ability to transmit signals from the subscriber receivers to the cable head end. These return transmissions are typically accomplished at a minimal data rate via a separate out-of-band signal.
It would be of significant advantage for a cable system operator to have knowledge of the actual signal levels being received at the various subscriber stations of the cable system. Such information can be used as a powerful diagnostic tool to isolate regions of low signal to noise (S/N) ratios, pinpoint defective amplifiers in the system, etc.
The known signal injection system can be used (with additional appropriate circuitry) at the subscriber stations, but such would be very expensive to implement. Further, with the coming use of digital video signals in cable systems, injecting a sweep signal across the cable band would be highly objectionable because significant portions of data would be lost.
Recently, Zenith developed and proposed for terrestrial standardization a multilevel VSB transmission system, for example, 8-level data and 2-level sync. For cable use, the system is 2/4/8/16 VSB because of the relatively benign cable environment. In accordance with the invention, this VSB system does not require the injection of a test signal for performing maintenance on a two way cable system. Further, in accordance with the invention, a two way cable system can readily have the capability of automatically monitoring the amplifiers in subscriber station cable boxes and detecting faulty or poor signal sites with a simple personal computer. This may be accomplished because the Zenith VSB transmission system includes a robust data component, specifically a two level training sequence that is used for equalization purposes. (There are also two level sync symbols and a small pilot signal in the Zenith signal.) The pilot insures carrier recovery independent of data and the use of both segment sync and frame sync signals permits symbol clock recovery and channel equalization even under conditions when the video data itself is unusable. The carrier sync clock recovery can occur at signal to noise (or carrier-to-noise) ratios as low as 0 dB. Channel equalization, which is accomplished with the two level training sequence data, can occur at S/N ratios close to the noise threshold of two level data, i.e. 2 VSB. Typically cable systems provide a carrier-to-noise (C/N) ratio of better than 40 dB and consequently a cable system incorporating a robust data component, according to the invention, in its signal and having appropriate receivers can provide significant diagnostic information despite failures or conditions that result in extremely poor or unusable signals. For example, a trunk or distribution amplifier with bridging will result in the signal being reduced in level.
While the inventive system just described uses the Zenith VSB transmission system, those skilled in the art will readily perceive that any video signal that includes a robust data component may be used to provide the self monitoring features of the invention.